Camouflage is necessary for deception and is often used by both animals and humans for disguise and protection. Camouflage techniques for the military have been pursued for well over a century but have primarily taken the form of surface colors and textures chosen for the particular milieu. In addition to personnel and land-based forces using these techniques, naval and aviation applications have been used since WWI. Coatings have ranged from neutral colors to razzle-dazzle schemes that break up the outline of large surfaces making it difficult to see the shape of the object. A variety of coloring schemes have been used aboard aircraft for years to provide delay of observation during daylight sorties. The Compass Ghost program during the Vietnam War is one such example.
Beginning in WWII however, a new technique was developed that is now generally termed active camouflage. The addition of energized lighting or display surfaces has been tested but rarely deployed even though shown to be successful in principle. This has the benefit of making the object not appearing to simply be a shadow. Through the use of surface illumination, an object can be made to substantially integrate with its surroundings, making it difficult to see with the eye.
During WWII, The US Navy's Project Yehudi used lights mounted on the leading edges of the wings of a torpedo bomber to successfully hide the plane in broad daylight when attacking a submarine. Visual detection range in the tests dropped substantially from 12 to 2 miles. As the plane approached a target, the lights, which pointed forward, were coupled with a photocell such that the output intensity (not color) of the light was set to match the intensity of the sky behind the approaching plane. This effect takes advantage of a physiological phenomenon termed isoluminance where objects of similar intensity can be indistinguishable from one another under certain conditions.
Yehudi, kept secret for many years, was never used because the advent of airborne radar systems in WWII rendered it moot. During the Vietnam War, however, a program called Compass Ghost revived advanced paint schemes and an attempt to try the Yehudi technique again on an F-4 Phantom. More recently in the mid 1990's were reports of a Project Ivy done by the Air Force that considered or used color panels.
The rapid development and deployment of radar systems combined with the end of the war eliminated the need for such techniques. The electromagnetic techniques of radio ranging through radar meant that eyes were trained upon radar displays and not the sky, and made pointless the need for such developments.
In the 1970s and 80's though, new developments in stealth aircraft rendered these aviation developments invisible to radar systems. Strikingly, although the stealth aircraft are nearly invisible to radar, they operate only at night because they are among the most visible of aircraft during the day.